Chromium plated steel sheet having an almost colorless and transparent chromate film



United States Patent 3,479,162 CHROMIUM PLATED STEEL SHEET HAVING AN ALMOST COLORLESS AND TRANSPARENT 'CHROMATE FILM Osamu Yanabu, Takashi Hada, and Kazuyuki Doi, Himeji-shi, Japan, assignors to Fuji Iron & Steel Co., Ltd., Tokyo, Japan No Drawing. Filed Mar. 23, 1967, Ser. No. 625,306 Claims priority, application Japan, Mar. 28, 1966, 41/19,141; Mar. 15, 1967, 42/16,287 Int. Cl. C23b 9/00; C22c 39/00 US. Cl. 29-195 9 Claims ABSTRACT OF THE DISCLOSURE A process for surface treating chromium plated steel sheet obtained by applying chromium plating directly on a single sheet or strip of carbon steel which comprises: electrolytically contacting said chromium plated steel as the cathode with an aqueous solution of chromic anhydride as the main ingredient and a hydroxide or carbonate of an alkali metal, calcium, magnesium or strontium to form an almost colorless and transparent chromate film on the chromium plating. The film enhances corrosion resistance and does not interfere with any subsequent painting of the plated steel.

This invention relates to a chromium plated steel sheet which is produced by forming directly on the surface of the steel sheet a plated layer of chromium of a thickness of about 0.00l0.1 and then coating the surface of the chromium layer with an almost colorless and transparent anticorrosive chromate film. This invention also relates to a chromate coating process for a chromium plated steel plate in order to form an almost colorless and transparent chromate film.

The purpose of this invention is to impart to the chromium plated steel sheet improved anticorrosi-ve properties, to retain the fine metallic luster peculiar to chromium, and not inhibiting color effects when painted.

Chromium plated steel sheet wherein the surface is coated with a thin layer of chromium is described in US. Patent No. 3,113,845 by the present inventors. Such a coating might be deteriorated by the presence of socalled pinholes. Therefore, a chromate coating on the plated layer is desirable. Investigations by the inventors on chromate coating processes, including dipping and electrolysis processes using various treating solutions, revealed that the anticorrosive property was remarkably improved by electrolysis, but much less by the dipping process. Nevertheless the electrolysis process using the conventional chromate coating liquid which contains chromic anhydride as a major constituent produces a colored opaque film on the plated layer of chromium. Consequently the fine metallic luster peculiar to chromium is lost and the appearance is spoiled. In addition, if the plate is painted, the color effect will be damaged. When paint, for example oily, phenolic, epoxyl, alky-d or vinyl 3,479,162 Patented Nov. 18, 1 969 ice paint, varnishes or lacquers, is applied, alone or in a mixture, on the colored chromate film produced by electrolysis, the bright color of the paint will be spoiled by interference with the coloration of the chromate film. Repeated applications of paint to avoid the interference of color necessarily decreases the working efliciency and is not recommended from the economical standpoint.

The present inventors, by the detailed investigation of the above difficulties, succeeded in the complete solution of the problem by making the chromate film colorless and transparent.

The present invention relates to a method for obtaining such colorless and transparent chromate film. It particularly relates to a method for surface treating a chromium plated steel sheet which is characterized by cathodically treating in an aqueous solution a chromium plated steel plate which has been obtained by applying a thin chromium coating directly on a single sheet or strip of ordinary carbon steel. The aqueous solution contains chromic anhydride as main component and one or more compounds selected from hydroxides or carbonates of alkali metals, calcium, magnesium or strontium in an amount greater than 20% by weight of the chromic acid.

Alkali metal hydroxides employed in this invention include NaOH, KOH and LiOH. On the other hand alkaline earth metal hydroxides includes Ca(OH) In addition to the above hydroxides, similar effects can be obtained by using carbonates which provide these hydroxides when added in the aqueous CrO solution, such as Na CO K2C03, Ligcog, Ca CO and SrCO The present invention relates to electrolytically treating a chromium plated steel sheet as cathode in an aqueous solution containing chromic anhydride as the main component with the addition of one or more of the above compounds in an amount more than 20% by weight of the chromic acid.

It has been known to cathodically or anodically treat a tin plate in an aqueous solution of sodium dichromate (Na Cr O or potassium dichromate (K Cr O It is considered that NaCr O or K Cr O will be produced by reaction of chromic anhydride with a chemical equivalent of NaOH or KOH. But various experiments conducted by the inventors shows that there is a large difference in the pH, as shown in Table 1, between an aqueous solution of Na2Cr207 and an aqueous solution of chromic anhydride with the addition of NaOH in a chemical equivalent amount to the chromic anhydride (40% by weight of the chromic anhydride). The solution of Na Cr O has a pH value between 4.0 and 4.5, while the latter solution has a pH value between about 1.7 and 2.8, depending upon the chromic anhydride concentration.

The present inventors found that there is a large difference in corrosion resistance between a chromium plated steel sheet which has been cathodically treated in an aqueous solution of Na Cr O or Kgcl'zoq and a chromium plated steel sheet which has been cathodically treated in an aqueous chromic anhydride solution with the addition of NaOH or KOH. The latter has superior corrosion resistance. The present inventors have also confirmed that this quality shows a similar relation in case of addition of hydroxides etc. of other alkali metals and alkali earth metals.

As for the aqueous solution containing chromic anhydride as main component, it may contain only chromic anhydride. In this case also, trivalent chromium is formed by the reduction of hexavalent chromium ion as the cathodic treatment process. Usually, this trivalent chromium ion is present within a range of 0.2 to g./l.

The cathodic treatment can be accelerate-d by adding to the aqueous solution of chromic anhydride the following compounds as subsidiary additives which act as catalyst for accelerating the cathodic treatment.

(1) Soluble compounds containing halogen (NaCl, H010 Na SiF (NHQ TiF CrCl etc.).

(2) Compounds containing certain anion (NaNO Na SO $11804, CI2(SO4)3 CtC.).

(3) Aromatic sulfonic acids (C H SO H.OH,

etc.).

(4) Soluble salts of metals (sodium stannate, sodium aluminate etc.).

Each of Cl-, C10 SiF TiF of the additives of the group 1), NO SO of the additives of the group (2), C H SO H, C H (SO H) of the additives of the groups (3) and Sn(OH) AlO of the additives of the group (4) produces a catalytic effect.

For utilization of these additives in the present invention, care should be taken to prevent precipitates caused by ions of the additives when hydroxides or carbonates of Ca or Sr are used.

It is preferable to avoid formation of such precipitates, although such precipitates are not formed even when hydroxides or carbonates of Ca or Sr are added in a considerable amount since the CrO solution has strong acidity and these additives are added in a small amount, less than 3%. Care should be taken particularly when the sulfate ion is added.

Now, there will be described an optimum content of hydroxides and carbonates of alkali metals, calcium, magnesium and strontium. When the concentration of chromic anhydride is within a range of to 60 g./l., the amount to be added of these hydroxides and carbonates must be more than 20% at least by weight of CrO and an amount less than 20% has no effect to prevent coloring of the chromate film.

As the CrO concentration decreases from 20 g./l., a smaller addition will produce adequate effect and good effect can be obtained if the addition is made in an amount more than 10% by weight of CrO Also as the CrO concentration increases above 60 g./l., the addition of these hydroxides and carbonates in an amount less than 20% by weight will be effective and good effect can be obtained if they are added in an amount more than 10% by weight of CrO Now considering this matter when these hydroxides and carbonates are added in a larger amount, coloring of the chromate film is effectively prevented through the range in which the precipitate is not formed. And in practice the addition of up to about 100% by weight of chromic anhydride will not adversely affect the corrosion resistance.

In the next place, the electrolytical chromate coating process is carried out under the following condition. The smallest possible concentration of chromic anhydride is about 5 g./l., because the anticorrosive property is decreased at too small a concentration. At very high concentrations, on the contrary, the anticorrosive property can not be substantially improved, therefore the upper limit of the concentration of chromic anhydride is decided from the point of economy.

It has been proved that a CrO concentration within a range of 5 g./l. to 300 g./l. is effective, but in practice a concentration from 50 g./l. to about 100 g./l. is used, and in this case the addition of hydroxides and carbonates of alkali metals, calcium, magnesium or strontium in an amount of 20% to 50% by weight of CrO is adequate.

Current density and the duration of electrolysis correlate to each other. Thus high current density requires only a short time of electrolysis and lower current density requires a longer time of electrolysis. Under the optimum condition, the current density is kept in the range 1 to 45 A./dm. and the time of electrolysis 0.1 to 15 sec.

The chromate treatment of the present invention is not substantially influenced by temperature. But at higher temperatures where the aqueous solution is gradually concentrated by evaporation difficulties are often encountered in maintaining the constant temperature and composition of the liquid in which the electrolysis takes place. Consequently the range from room temperature up to approximately C. is considered to be adequate.

The composition, structure and the process of formation of the almost colorless and transparent chromate coating film of this invention are not known. Nonetheless the electrolysis of the chromium plated steel sheet as cathode using the chromate coating liquid of this invention can give a colorless and transparent chromate coating film of excellent anticorrosive property, in contrast with the colored chromate films which are formed when conventional chromate coating solutions are used as electrolyte. Thus the almost colorless and transparent product of this invention provides high anticorrosive properties and remarkably enhanced commercial value.

Advantages of the present invention are that the fine metallic luster, peculiar to chromium, of the base is not spoiled and that, when the plate is painted, the bright color of the paint is not deleteriously affected.

This is especially noteworthy with a white coating. Taking a white coating with an alkyd paint for example, single application (15 suffices for the effect onto the colorless and transparent chromate film of this invention, while at least two applications are needed onto colored chromate films.

The single operation of painting improves the working efficiency and the thin layer of paint costs less.

The chromate coating of a steel plate covered with a thin layer of plated chromium by the electrolysis process as cathode was comparatively examined relative to the coating by the dipping process. With the anticorrosive property, examined by the saline water spray test, 118 Z2371, the chromate coating of the latter process allowed rust to appear usually in several hours, while with the coating of the former process it took well over ten hours. More particularly rust developed in spots where the pinholes existed in the latter process, while in the former where the chromate coating film was worn out. This fact suggested that the former process produced much stronger and more compact chromate coating films. This is the reason why the thickness of the plated layer of chromium can be reduced, in case of coating by electrolysis, down to 0.00m as the lower limit. The upper limit of thickness may be about 0.1/L for the ease of working.

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As shown in the above example the other process No. 1, is not effective for preventing coloring of the chromate film when NaOH is in an amount of 10% by weight of tion, but almost colorless and transparent chromate film CrO in case of 20 g./l. to 60 g./l. of CrO concentrais obtained when NaOH is in an amount of 22% by weight of CrO as shown in Example No. 10 of the present invention. Also as shown in the present invention Example No. 19, the chromate film is almost colorless and transparent in case the CrO concentration is less than 20 g./l. even when the amount of NaOH is 12% by weight of CrO But in this case, as shown in Example No. of the electrolytical colored chromate treatment, it is not effective for preventing coloring of the chromate film when the amount of NaOH is 5% by weight of CTO3.

Further, the chromate film is almost colorless and transparent, as shown in Example No. 12 of the present invention, even when the amount of NaOH is 12% by weight of CrO in case the CrO concentration is more than 60 g./l. In this case also, when the amount of NaOH is less than 5% by weight of CrO coloration takes place, as shown in Example No. 6 of the electrolytical colored chromate treatment.

Meanwhile, as shown in Example No. 2 of the other process, a chromium plated steel sheet which has been cathodically treated in an aqueous solution of Na Cr O has inferior corrosion resistance, whereas in case of Ex ample No. 12 of the present invention where NaOH and CrO are in a chemical equivalent relation, the corrosion resistance is good showing more than 20 hours by the salt spray testing.

The process of this invention forms a colorless and transparent chromate coating film without any loss in the anticorrosive property on a steel plate covered with a thin layer of plated chromium. As is evident in the above examples, the advantages of the colorless and transparent coating film are that the appearance can be remarkably improved by the metallic luster peculiar to chromium, and that, when a white coating is applied with an epoxy paint, a single application can produce a satisfactory color effect, while in contrast a colored chromate coating film requires more than two applications for good color effect. The advantages, of course, considerably enhance the commercial value of the product. In addition, when the product of this invention is painted, the working efficiency can be improved owing to the single application and the consequent thin layer of paint is economical.

What is claimed is:

1. A process for surface treating chromium plated steel sheet which has been obtained by applying chromium plating directly on a single sheet or strip of carbon steel which comprises: electrolytically contacting said chromium plated steel as the cathode with an aqueous solution of chromic anhydride in a concentration of 5 grams per liter to 300 grams per liter of said solution, said solution further containing hydroxides, or carbonates which convert to hydroxides in said solution, of an alkali metal, calcium, magnesium or strontium, and wherein said solution contains (a) greater than 20% of said hydroxide or carbonate, based on the quantity of chromic anhydride,

when the concentration of chromic anhydride is in the range of 20 to grams per liter of solution, or (b) at least 10% of said hydroxide or carbonate, based on the quantity of chromic anhydride, when the concentration of chromic anhydride is below 20 grams per liter or above 60 grams per liter of said solution.

2. A process of claim 1 wherein the aqueous solution further contains one or more ions selected from the group consisting of Cl, C10 1 SiF TiFy, N0 1 C H SO H, C H (SO H) Sn(OH) and A1 0.

3. A process of claim 1 wherein the concentration of chromic anhydride is from 20 to 60 grams per liter of the aqueous solution and the amount of said hydroxide or carbonate is over 20% by weight of the chromic anhydride.

4. A process of claim 1 wherein the concentration of chromic anhydride is (a) below 20 grams per liter or (b) above 60 grams per liter of said aqueous solution, and said solution contains at least 10% of said hydroxide or carbonate, based on the quantity of chromic anhydride.

5. A process of claim 1 wherein the aqueous solution contains from 0.2 to 10 grams per liter of trivalent chromium ion.

6. A process of claim 1 wherein the electrolytic contact of the chromium plated steel with said solution is effected at a current density of 1 to 45 A per dm. for 0.1 to 15 seconds and wherein the temperature of said solution is from room temperature to C.

7. A process for surface treating chromium plated steel sheet which has been obtained by applying chromium plating directly on a single sheet or strip of carbon steel which comprises: electrolytically contacting said chromium plated steel as the cathode with an aqueous solution of chromic anhydride containing (a) chromic anhydride in a concentration of from 20 to 60 grams per liter and (b) a hydroxide or carbonate of an alkali metal or calcium, magnesium or strontium in a concentration of from above 20% to based on the weight of said chromic anhydride and wherein said electrolytic contact is effected at a current density of 1 to 20 A per dm. for 0.1 to 7 seconds at a solution temperature of 40 to 65 C.

8. A process of claim 7 wherein said aqueous solution further contains from 0.1 to 5 grams per liter of SiF and the alkali metal is sodium.

9. Chromium plated steel sheet having an almost colorless and transparent corrosion resistant chromate film when prepared by the process of claim 1.

References Cited UNITED STATES PATENTS 8/1961 Kitamura 20456 5/ 1962 Yonezaki et al. 20456 US. Cl. X.R. 204 41 

